1. Field of the Invention
This present invention relates to molten metal electrodes in an operating, rechargeable oxide-ion battery (ROB) cell.
2. Description of Related Art
Electrical energy storage is crucial for the effective proliferation of an electrical economy and for the implementation of many renewable energy technologies. During the past two decades, the demand for the storage of electrical energy has increased significantly in the areas of portable, transportation, load-leveling and central backup applications. The present electrochemical energy storage systems are too costly to penetrate major new markets. Higher performance is required, and environmentally acceptable materials are preferred. Transformational changes in electrical energy storage science and technology are in great demand to allow higher and faster energy storage at lower costs, and longer lifetimes are necessary for major market enlargement. Most of these changes require new materials and/or innovative concepts, with demonstration of larger redox (reduction-oxidation) and reversible and facile kinetics.
Batteries are by far the most common form of storing electrical energy ranging from: standard every day lead-acid batteries; exotic iron-silver batteries taught by Brown in U.S. Pat. No. 4,078,125; nickel-metal hydride (NiMH) batteries taught by Kitayama in U.S. Pat. No. 6,399,247 B1; metal-air batteries taught in U.S. Pat. No. 3,977,901 (Buzzelli) and Isenberg in U.S. Pat. No. 4,054,729; and to the lithium-ion battery taught by Ohata in U.S. Pat. No. 7,396,612 B2.
Batteries range in size from button cells used in watches, to megawatt load leveling applications. They are, in general, efficient storage devices, with output energy typically exceeding 90% of input energy, except at the highest power densities. Rechargeable batteries have evolved over the years from lead-acid through nickel-cadmium and nickel-metal hydride (NiMH) to lithium-ion batteries. NiMH batteries were the initial workhorse for electronic devices such as computers and cell phones, but they have almost been completely displaced from that market by lithium-ion batteries because of the latter's higher energy storage capacity. Today, NiMH technology is the principal battery used in hybrid electric vehicles, but it is likely to be displaced by the higher power energy and now lower cost lithium batteries, if the latter's safety and lifetime can be improved. Of the advanced batteries, lithium-ion is the dominant power source for most rechargeable electronic devices.
What is needed is a dramatically new electrical energy storage device that can easily discharge and charge a high capacity of energy quickly and reversibly, as needed. What is also needed is a device that can operate safely. What is also needed is a device that can operate for years without major maintenance. What is also needed is a device that does not need to operate on natural gas, hydrocarbon fuel or its reformed by-products such as H2. One possibility is a rechargeable oxide-ion battery (ROB), as set out application Ser. No. 12/695,386, filed on Jan. 28, 2010.
A ROB comprises a metal electrode, an oxide-ion conductive electrolyte, and a cathode. The metal electrode undergoes reduction-oxidation cycles during charge and discharge processes for energy storage. For example, in discharging mode, the metal is oxidized:yMe+x/2O2=MeyOx and is reduced in charging mode:MeyOx=x/2O2+yMe, where Me=metal.
The working principles of a rechargeable oxide-ion battery (ROB) cell 10 are schematically shown in FIG. 1. In discharge mode, oxide-ion anions migrate from high partial pressure of oxygen side air electrode—12 to low partial pressure of oxygen side metal electrode—14 under the driving force of gradient of oxygen chemical potential. There exist two possible reaction mechanisms to oxidize the metal, as shown in FIG. 1. One of them, as designated as Path 1, in that oxide ion can directly electrochemically oxidize metal to form metal oxide. The other, as designated as Path 2, involves generation and consumption of gaseous phase oxygen. The oxide ion can be initially converted to gaseous oxygen molecule on the metal electrode 14, and then further reacted with metal via a solid-gas phase mechanism to form metal oxide. In charge mode, the oxygen species, released by reducing metal oxide to metal via electrochemical Path 1 or solid-gas mechanism Path 2, are transported from the metal electrode back to the air electrode. The electrolyte is shown as 16.
The metal redox reactions are accompanied by large volume variation, for instance, if manganese (Mn) metal is used as the metal electrode, the volume change associated with reaction of Mn+½O2=MnO is 1.73. In the case of tungsten (W), the volume change is 3.39 when W is totally oxidized to WO3. Without an appropriately designed electrode, such drastic volume variation in practice can lead to spallation of metal electrode and possible failure of a ROB cell.
In theory, for energy storage application, oxide ion must be transported across the electrolyte between metal electrode and cathode to carry electrical charge. Therefore, the metal electrode must be properly sealed hermetically to prevent direct contact with an oxygen-containing environment (for example, air). Otherwise, oxygen molecules in air will directly consume the metal without involving charge transfer between electrodes, which will lead to self discharge. High-temperature sealing materials such as glasses and ceramic-glass composites in principle are good candidates for this purpose. However, reliability of high-temperature sealing materials remains questionable upon thermal cycle and long-term operation. Therefore, there is need to design ROB cells whose metal electrode is sealed solely by the cell electrolyte and interconnection without using additional materials for sealing purpose.
Additionally, one of the major challenges facing rechargeable oxide-ion batteries (ROB) are the solid-state oxygen diffusion through the oxide layer and volumetric changes associated with oxidation and reduction during the charging and discharging cycles. In general, in one aspect of application Ser. No. 12/695,386, filed on Jan. 28, 2010, this challenge was addressed by incorporating solid, fine, active metal electrode powders into a mixed oxide-ion and electron conducting porous structural skeleton. It was conceivable that the changes in volume induced during operation could be accommodated by the free volume present in the pores. In such a battery structure, the corresponding electrochemical reactions are perceived to occur via gas-phase as well as solid-phase oxygen diffusion. Despite these engineering design efforts, the fairly slow kinetics of solid-state oxygen diffusion, particularly at lower temperatures is a possible issue for making ROB a practical energy storage device. Therefore, an effective solution to this challenge is deemed a critical path to ROB technology and product development. It is an object of this invention to provide a faster kinetics and a new active metal electrode design for ROB.